This invention is in the field of reducing or eliminating acid rock drainage from sulfidic iron containing rocks and acidic mine waste tailings. Acid rock drainage (formation of sulfuric acid and related acids from natural air/water oxidation processes on various materials) is a common phenomenon from mining and leaching of various metallic and non-metallic minerals such as iron-containing sulfidic materials. These sulfidic materials include tailings, overburden, discarded waste rock and unmined exposed rock. Acid rock drainage causes severe pollution problems throughout the world.
More specifically, in mining operations large amounts of rock containing sulfide minerals are typically excavated in an open pit or opened up in an underground mine. This rock, in turn, can react with water and oxygen to produce sulfuric acid and related acids. When the water reaches a certain level of acidity, naturally-occurring bacteria called Thiobacillus feroxidans may become active and accelerate the oxidation and acidification processes, leaching even more trace metals from the wastes. The acid will leach from the rock as long as its source rock is exposed to air and water and until the sulfides are leached out—a process that can last hundreds to thousands of years. Rainwater or surface drainage can carry acid from the minesite and deposit it into nearby streams, lakes, rivers, and groundwater. Acid rock drainage (AD) severely degrades surface and groundwater quality and can strongly affect the ecosystems of lakes, streams, and estuaries.
Heavy metal pollution is caused when such metals as cobalt, cadmium, lead, copper, silver, zinc, and arsenic that can occur naturally in excavated rock are exposed in a mine and come in contact with water. As water flows over the rock surface, metals can be leached out and carried downstream. Although metals can become mobile in neutral pH conditions, leaching is particularly accelerated in the low pH conditions that are created by acid rock drainage.
According to U.S. Bureau of Mines estimates in 1989, coal and metal mines and the associated piles of mine wastes alone adversely affected over 19,000 kilometers of rivers and streams and over 73,000 hectares of lakes and reservoirs in the United States. The prevention and treatment of AD requires large sums of money. In one of the largest coal-producing states in the country, West Virginia, approximately $350 million per year is spent to treat AD. In addition to direct costs, there are additional costs that are attributed to diminishing land and water quality.
Mining activities at base and precious metal, uranium, diamond, and coal mines produce acid drainage by the oxidation of sulfide minerals, primarily pyrite (FeS2) and marcasite (FeS2). Other important metal sulfides which can occur in mining regions can include:
MoS2molybdeniteFexSxpyrrhotiteCu2SchalcociteCuScovelliteCuFeS2chalcopyriteNiSmilleritePbSgalenaZnSsphaleriteFeAsSarsenopyrite
Sulfide minerals oxidize to form highly acidic sulfate-rich drainage in the presence of oxygen and water. Releases of acid drainage in the environment occur as runoff or seepage from waste rock stockpiles, tailings impoundments, spent heap leach ore and open pit walls or as groundwater discharge from mine adits.
The chemical reactions governing the oxidation of pyrite and subsequent acid generation have been presented as:
                                          FeS                          2              ⁢                              (                s                )                                              +                                    7              2                        ⁢                          O              2                                +                                    H              2                        ⁢            O                          →                              Fe                          (              aq              )                                      +              2                                +                      2            ⁢                          SO                              4                ⁢                                                                  ⁢                                  (                  aq                  )                                                            -                2                                              +                      2            ⁢                          H              +                                                          (        1        )                                                      Fe                          (              aq              )                                      +              2                                +                                    1              4                        ⁢                          O              2                                +                      H            +                          →                              Fe                          (              aq              )                                      +              3                                +                                    1              2                        ⁢                          H              2                        ⁢            O                                              (        2        )            Fe+3(aq)+3H2O→Fe(OH)3(s)+3H+  (3)FeS2(s)+14Fe+3(aq)+8H2O→15Fe+2(aq)+2SO4−2(aq) +16H+  (4)
Equation 2 describes the oxidation of ferrous (Fe+2) to ferric (Fe+3) iron and is also the rate-limiting step in the oxidation of pyrite. Thiobacillus ferroxidans and probably other oxidizing bacteria act as catalysts during this reaction, increasing overall oxidation rates of ferrous iron by several orders of magnitude. Once ferrous iron is oxidized to ferric iron, the ferric ion can react with pyrite in Equation 4 to produce greater amounts of acidity than Equation 1 in which oxygen is the oxidizing agent.
Many approaches have been suggested to solve the AD problem. These approaches include using techniques to eliminate oxygen, sulfides or water to control acid generation at its source; encapsulating or coating the pyrite; using bactericides; neutralizing acid drainage from groundwater and surface water sources using limestone, quicklime or slaked lime; creating aerobic or anaerobic wetlands to treat acidic water; and using diversion wells or open limestone channels to passively treat AD. These methods of treating materials have been at best only partially effective and economically unattractive.
U.S. Pat. No. 5,587,001 (DeVries, Dec. 24, 1996) describes a method for reducing acid rock drainage from sulfidic iron-containing rock by contacting the rock with an aqueous solution of permanganate ion at a pH between 6-13. This treatment reportedly creates a manganese oxide layer on the iron-containing sulfidic rock. The process in the U.S. Pat. No. 5,587,001 requires pH 6-13 at all times during the treatment, preferably a pH greater than 10. U.S. Pat. No. 5,587,001 also requires that permanganata color be maintained during the treatment. This condition often requires high dosage of permanganate ions for treating reactive tailings because a considerable amount of permanganate ions are dissolved in solution and react with other ions before reaching the sulfide surface. U.S. Pat. No. 5,587,001 also requires that the sulfides contain a significant concentration of iron bearing minerals so that the reaction between iron bearing sulfides and permanganate ions can be sustained. Several dissolved metals undergo precipitation reactions at pH>12. Precipitated metal hydroxycomplexes coat the sulfides, thus preventing the desired electrochemical reaction. Some problems associated with this process involve the addition of large amounts of lime to maintain a pH of 12. The amount of potassium permanganate required to form a stable coating on sulfide minerals in the treatment of waste rocks and tailings can result in high costs. Further, there is the problem of working with a very strong oxidant.
U.S. Pat. No. 6,086,847 (Thompson, Jul. 11, 2000) discloses a process for reportedly preventing acid rock drainage of metal-bearing rocks comprising contacting a sulfidic iron-containing rock with an acid passivating agent which comprises at least one alkaline earth metal to produce a combination; contacting the combination with manganate ions and a base and maintaining the pH of the system between 11 and 13.5.
The waste rock naturally yields very low acidic pH in the range of 1-4. To raise the pH and maintain it at a higher level than is naturally found (such as the pH required by the process disclosed in U.S. Pat. Nos. 5,587,001 and 6,086,847) requires high dosage of neutralization agents (for example, lime/caustic soda). This is not economically and technically viable. Also, at high pH (above about 11.0), gypsum (CaSO4) and MgSO4 precipitate on the sulfide and affect the coating of desired materials. Improved and cost effective treatments are necessary to treat iron containing sulfidic minerals to prevent or minimize the natural oxidation of these materials to form acids.